There is a continuing need for methods of modulating male fertility. For example, in humans, there is a need for alternatives to surgical approaches to male contraception such as castration and vasectomy. There is also a new and growing need for methods of inhibiting fertility in genetically modified organisms, as a means for preventing the dissemination of genetic modifications into wild-type populations.
Hormone-sensitive lipase (E.C. 3.1.1.3) has several substrates, all of which are fatty acyl esters. Hormone-sensitive lipase cleaves fatty acids from triacylglycerides and diacylglycerides at the 1- and 3-positions, cholesteryl esters and esters of retinoic acid and of steroid hormones. Hormone-sensitive lipase is active in adipose tissue, where it plays an important role in the release of fatty acids (Langin et al., 1996, Proceeding of the Nutrition Society 55:93). Hormone-sensitive lipase is also expressed in many non-adipose tissues, such as adrenal gland, pancreatic beta cells, macrophages and testicles (Holst et al., 1996, Genomics 35:441; Mulder et al., 1999, Diabestes 48:228; Reue et al., 1997, Arterioscler Thromb Basic Biol 17:3428; Blaise et al., 1999, J. Biol. Chem 274:9327).
The human hormone-sensitive lipase gene has been cloned (Langin et al., 1993, Proc. Natl. Acad. Sci USA 90:4897). Hormone-sensitive lipase is an amphiphilic 84 Kda protein with little homology to other mammalian lipases, containing three residues thought to be essential for serine protease activity (Ser 424, Asp 693 and His 723). In testes, hormone-sensitive lipase is expressed as a 130 KDa isoform, for which transcription begins in a specific upstream exon that is spliced to a site 22 base pairs upstream of the initiation methionine codon of nontesticular transcripts. The testicular hormone-sensitive lipase isoform therefore contains all of the sequence of nontesticular hormone-sensitive lipase plus an N-terminal extension encoded by the first (testes specific) exon. The testicular form of hormone-sensitive lipase is expressed at a specific phase of sperm development, in round and elongating spermatids.
A number of small molecule inhibitors of hormone-sensitive lipase are known, as shown in Table 1.
TABLE 1Inhibitors of hormone-sensitive lipaseInhibitorIC50*Diisopropylfluorophosphate (DFP)9 microMHgCl2 (mercuric chloride)11 microMNaF (sodium fluoride)25 Mm*concentration producing 50% inhibition (Stralfors et al., 1987, The Enzymes, Vol. XVIII, p. 147–177, Academic Press)